Well I might just as well suck it up and admit it.
I NEVER knew how to ride my bike until about 2 years ago. Never learned. And as an adult, yes, there's a lot of pride on the line. I recommend starting with something cheap and disposable (just in case it's not the thing for your friend, no harm done), as long as you adjust it so that the majority of the parts work, that's fine.
Put it in a gear where you can pedal at around 60-70 rpm at around 10mph, no more to start with and never touch it again. Either disable or rotate the front brake all the way down so he won't touch it, rely on the rear brake initially until he can control the bike, then teach him to use the front properly. Lower the seat so it's easy to get the feet on the ground if he goes off balance. Distance matters a lot, do not have him practice in a small lot, you need enough distance to get up to speed (1/8 mile at least) in order to really balance well. Wobbling around in an empty lot won't help. Have him wear knee pads under his clothes, he will most likely fall a few times as I did. Do not have him wobbling around at 2mph. Try to have him push off with his feet and "coast" the bike as far as possible with his feet off the ground holding onto the handle bars. With enough distance and speed, the bike should almost balance by itself (it took me about a year to realize this and be able to ride no handed down the street).

The most important thing is a patient teacher, especially for an adult. If his pride is shot, learning becomes next to impossible. In my case, my gf was patient enough to stay with me for about 2 afternoons, which is about how long it took me to learn how to ride straight and steer w/o falling. I started with a cheap walmart bike and within about a month, I tried a 5 mile commute. 3 months later, my gf bought me a specialized hardrock for a present and 6 months later, I had increased it to a 10 mile round trip commute, bought my first road bike. A year later, I was doing a 25 mile round trip commute. Now I have 3 bikes in my room and I'm rapidly running out of space to walk.

I guess try to curb his enthusiasm so he doesn't end up like me.